5th Swiss Geoscience Meeting, Geneva 2007
Origin of Neoproterozoic detrital zircon in the ProtoAndes
David Chew*, Tomas Magna**, Chris Kirkland***, Aleksandar Miskovic**** &
Urs Schaltegger****
*Department of Geology, Trinity College Dublin, Dublin 2, Ireland (chewd@tcd.ie)
**Institute of Mineralogy and Geochemistry, University of Lausanne, CH-1015
Lausanne (Tomas.Magna@unil.ch)
*** Laboratory for Isotope Geology, Swedish Museum of Natural History, S-104 05
Stockholm, Sweden (Chris.Kirkland@nrm.se)
****Department of Mineralogy, University of Geneva, Rue de Maraîchers 13, CH-1205
Geneva (urs.schaltegger@terre.unige.ch)
The detrital zircon record of clastic sediments is a powerful provenance tool
which can link sedimentary basins and potential source regions. We provide
examples of detrital zircon populations from autochthonous rocks from the
northern and central segments of the Proto-Andean margin of South America,
which formed part of the western margin of Gondwana during the Late
Neoproterozoic - Palaeozoic. The Proto-Andean margin can be demonstrated
to be the source region for most analyzed samples. However, a small but
significant subpopulation in the detrital zircon record (in the 0.55 – 0.85 Ga age
range) is extremely common in samples from the Proto-Andean margin from
Ecuador to Southern Argentina. In the northern and central Andes there is no
obvious source for this detritus, and we suggest that a magmatic belt of this age
is buried underneath the present-day Andean belt.
The detrital zircon data were obtained from both sedimentary rocks and from
xenocrystic cores in S-type granites and migmatitic rocks from the northern and
central Andes. These inherited zircon cores offer additional provenance
information, as they may transport xenocrystic zircon from deeper crustal levels
that would otherwise be impossible to sample. We compile data from 4 new
samples and 13 recently published samples from the literature (Chew et al. in
press, Chew et al., 2007, Cardona et al., 2006), the sedimentary samples
spanning a broad range of depositional ages (Late Neoproterozoic to
Cretaceous), while the crystallization ages of the magmatic samples range from
the Late Ordovician to the Middle Triassic. None of these sedimentary
sequences, or any of the plutonic bodies, has been clearly demonstrated to be
exotic to the Proto-Andean margin (Chew et al., 2007).
The majority of samples exhibit a prominent peak between 0.45 and 0.5 Ga,
and the most likely source is a subduction-related magmatic belt in the Eastern
Cordillera of Peru (Chew et al., 2007), similar to the Famatinian arc of northern
Argentina (Pankhurst et al., 1998). A second prominent peak is encountered
between 0.9 – 1.3 Ga. The most likely source for this population is the Sunsas
orogen of the Southwest Amazonian craton which probably lies beneath the
Northern and Central Andean foreland basins and is contiguous with the c. 1 Ga
gneissic basement inliers in the Columbian Andes (Restrepo-Pace et al., 1997).
Most samples yield minimal detritus older than 2 Ga.
5th Swiss Geoscience Meeting, Geneva 2007
However, the source of detritus in the 0.55 – 0.85 Ga age range is
problematical as there is little evidence on the western Gondwanan margin for
magmatism at this time. Although there is voluminous magmatism of this age in
the Brasiliano orogeny, this orogenic belt crops out on the eastern side of the
Amazonia craton. Therefore it is not considered a likely source for the majority
of the samples, as old detritus (i.e. > 2 Ga) which would be characteristic of the
core of the Amazonian craton is usually absent in the detrital zircon record.
On the proto-Andean margin, juvenile extensional magmatism (dacite dykes)
has been dated at 635 ± 4 Ma in the Arequipa – Antofalla Basement of Northern
Chile (Loewy et al., 2004), while A-type orthogneisses of mid Neoproterozoic
age (774 ± 6 Ma) have been documented from the Grenvillian basement of the
Precordillera terrane. This extension-related magmatism on the incipient ProtoAndean margin was probably produced by the rifting of Laurentia from the
Rodinian supercontinent, and is thought the most likely source of the 0.65 –
0.85 Ga detritus. The transition from an extensional tectonic regime to a
destructive margin (i.e. the initiation of the Proto-Andean margin which
remained an active margin for much of the Phanerozoic) in the northern and
central Andes is presently undefined, but may be recorded by a discrete pulse
in the detrital zircon record at around 0.55 Ga, contemporaneous with the onset
of subduction-related granitoid magmatism and high-grade metamorphism,
which initiated at ca. 530 Ma in the Sierra Pampeanas in northern Argentina
(Rapela et al., 1998).
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